1. Technical Field
The present disclosure relates generally to an apparatus and method for calibrating color display systems that use digital micro-mirror device (DMD) technology in projection image systems. More particularly, the present disclosure relates to a digital light processing (DLP) system that uses non-imaging light to monitor and calibrate the projected image, such that the apparatus and method are non-invasive and may be applied to existing DLP systems with minimal modification.
2. Background Art
Digital Light Processing™ and DLP™ technology (both trademarked by Texas Instruments, Inc.) refer to an all-digital display technology that is used in projectors and televisions. A DLP projection system is an image projection system comprising a powerful light source focused on a controlled multiple mirror element, with a lens assembly focusing on a plane defined by each mirror of the multiple mirror element and projecting the reflected light onto an image screen. The mirrors are controlled digitally to provide an on/off signal for each image pixel. This mirror assembly is referred to as the DMD, a semiconductor-based “light switch” array of thousands of individually addressable, tiltable, mirror-pixels. The DMD chip is a spatial light modulator (SLM) and brings many advantages to light-steering applications.
When a DMD chip is coordinated with a digital video or graphic signal, a light source, and a projection lens, mirrors of the DMD can reflect an all-digital image onto a screen or other surface. DLP systems include the DMD and surrounding electronics associated with it.
Each mirror of the DMD is one of thousands of tiny mirrors in an array and is attached to one or more hinges mounted on support posts permitting the mirror to tilt. Each mirror is spaced by means of an air gap over underlying addressing circuitry. The addressing circuitry provides electrostatic forces which cause each mirror to selectively tilt.
For display applications, the DMD is addressed with image data. In accordance with this image data, light is selectively reflected from each mirror and projected onto a viewing screen. The combination of light and dark mirrors forms an image. Modulation techniques are used to provide grayscale image “frames”. A quick succession of frames is perceived by the viewer as a full motion display.
There are at least two approaches to generating color displays with a DMD display system. One approach is to generate multiple images with multiple SLMs, typically one SLM each for red, green and blue. Each image has a desired intensity and the images are combined to result in a correctly colored display. A second approach is to use a single SLM and generate images for each color (red, green, and blue) sequentially. A white light source is filtered through a revolving color wheel, such that a desired color illuminates the corresponding image. The differently colored images are generated so quickly that the eye integrates them into a correctly colored frame.
When individual mirrors direct light through the imaging lens, the imaging surface is selectively illuminated. When the mirror reflects light away from the imaging lens, the reflected light is collected in an area referred to as a waste gate. DLP systems that use a single DMD element typically use a spinning color filter wheel to separate light (e.g., red, green, blue) into sequential color outputs. During the active time for a single color, the mirror elements are addressed to form an image pixel. The individual mirrors actively aim light thru the imaging lens for a time proportional to the intensity, which corresponds to a desired image pixel intensity. The appearance of a continuous change in intensity can be enhanced by techniques such as frame multiplexing and the addition of fixed pattern noise.
DLP technology made possible by the advent of DMD chips has led to the emergence of significant new projection display technology over the last decade. In addition to commercial success in high definition television (HDTV) and theater projection system areas, DLP technology finds potential in the areas of adaptive lighting, medical imaging, photo-finishing, biotechnology applications, lithography, spectroscopy, and scientific instrumentation, to name a few.
Most, if not all, of these applications can be improved significantly by the addition of a real-time and/or static calibration means to assure maintenance of the inherent high quality image in the presence of known factors of degradation, such as spectral and intensity changes in the light source. Furthermore, a calibration means is desirable to adjust initial settings of the system, including compensating for aging and degradation of system elements, such as the light source, for example. Such compensation would advantageously account for accumulation of dirt films on system optical elements and light losses due to physical changes in the system over time.
It is normal and desirable to place calibration elements within a processing system, in this case a DMD device. However, the projection technology in DLP systems requires that a significant amount of light and corresponding heat be directed onto a relatively small area of the DMD chip. The concentration of light and associated heat in this small area raises operating temperatures and becomes a hostile environment for semiconductor sensors used in such image calibration elements. The challenge then becomes one of how to effectively calibrate such devices.
The calibration of such products is normally done by measuring reflected light from or light incident to the projection surface. The assignee of the present disclosure has manufactured products that incorporate the use of a lens cap technique. The lens cap-based process produces good results, but requires a cap to be put in place prior to measurement. Furthermore, placement of the cap may be difficult when the projection system is mounted on a high ceiling or behind a rear-projection screen.
Accordingly, despite that which is known from the prior art, a need remains for an apparatus and method for calibration of DLP/DMD projection imaging systems having certain desirable features and functionalities. In particular, a need remains for an embedded calibration apparatus and method that is not negatively impacted by heat factors and that requires no interruption of the optical path.
These and other advantageous features, functionalities and/or capabilities are provided according to an exemplary calibration device, system and/or method for a DLP system that uses a digital micro-mirror device (DMD) for modulation of projected image intensity, as disclosed herein.